Cessna/Textron have built over 45,000 of them during those 70 years. No need to reinvent the wheel, when certification costs are crippling. Look at Ford for example. They’ve produced 40M+ F-150 trucks over 75 years. It still has a V-8 engine, 4 wheels, a bed, and a cab, even though the guts and tech have been upgraded a lot, just like the C-172.